Sunfish description

Not only is the sunfish (Mola mola) the world’s heaviest bony fish, with some individuals weighing in at a staggering 2.3 tonnes (3)(4), but it also possesses a truly bizarre body shape, likened to a gigantic ‘swimming head’ (3)(5). The sunfish has no tail, with the caudal fin reduced to a rudder-like structure, called the clavus. The dorsal and anal fins are placed far back on the body, and are used as ‘oars’ in swimming (the fins are flapped synchronously to propel the fish through the water) (2)(3)(5)(6)(7). The sunfish does not have scales, and instead has a tough, elastic skin which is covered in mucus. Also unlike other fish, it has fewer vertebrae, lacks bony tissue in the skeleton, and does not possess ribs, pelvic fins or a swim bladder(5)(6)(8). The mouth of the sunfish is small, and its teeth are fused together to form a ‘beak’ (6)(9). The sunfish tends to be brownish-blue to silver in colour, often with a slightly iridescent sheen, and it may also exhibit various patterns on the skin (4)(9).

Related species

Sunfish biology

The reproductive biology and behaviour of the sunfish are poorly understood. However, female sunfish are known to carry an extraordinary number of eggs, with an individual female capable of producing up to 300 million eggs at one time, the largest number of eggs ever recorded in a vertebrate(3)(4)(5)(8). Where and when the sunfish spawns is not well known, although five possible areas have been identified in the North and South Atlantic, the North and South Pacific, and in the Indian Ocean, where there are central rotating oceanic currents, called gyres (3)(5). The newly hatched sunfish measure just 0.25 centimetres in length, and will increase in mass by over 60 million times in order to reach the size of a 3 metre adult (3)(5)(8).

The sunfish is thought to feed mainly on jellyfish, and its diet may also include a variety of alternative prey species including crustaceans, molluscs, squid, small fish and deepwater eel larvae(2)(4)(5)(6)(8)(9). The frequently observed ‘basking’ behaviour, where the sunfish swims on its side at the ocean surface, is thought to be linked to the deep dives which it makes periodically throughout the day, and which are most likely made in search of prey (3)(5). Basking may be a form of thermoregulation, allowing the sunfish to warm up after making forays into cooler waters; however, other explanations for this peculiar behaviour include illness, or possibly the solicitation of cleaner fish or birds, for the removal of parasites(2)(3)(5)(9). Known to harbour many parasites, at least 40 different genera have been recorded using the sunfish as a host, and crustaceans have also been found attached to the skin and gills of many individuals (3)(4)(5).

Species with a similar range

Sunfish habitat

This marine species spends its entire life in the open ocean, and relatively little is known about its ecology. The sunfish is often sighted drifting at the surface (5)(6)(8), and it is likely that it spends the majority of its time in near-surface waters (5). The current view is that the sunfish inhabits the warmer layers of water at night, at depths of 12 to 50 metres (3)(7), but will make periodic dives below this level into cooler water during the day, usually to depths of around 40 to 150 metres (3)(5). The sunfish has also been recorded reaching depths of up to 500 metres below the surface, although it is not known whether this is typical diving behaviour (5).

Species found in a similar habitat

Sunfish status

Sunfish threats

The most significant threat to the sunfish is from fisheries, with the species comprising a huge proportion of bycatch in most fisheries that operate in the Pacific and Atlantic Oceans and in the Mediterranean Sea. The sunfish is not a commercially important fish, but in some areas of the world, it can make up as much as 90 percent of the total catch, and will often greatly outnumber the target species caught in many hauls (3)(5)(7). This is especially concerning if the sunfish exists in discrete populations, rather than one large global population, as small, localised populations are at much higher risk of depletion or extinction (3).

Sunfish conservation

There are currently no conservation measures in place to protect this species, and perhaps more worryingly, there is very little information on the basic biology of the sunfish, including its foraging and diving behaviour, population structure, and its distribution and seasonal movements throughout the world’s oceans (5). There is growing recognition among the scientific community that further research on the sunfish is essential, especially in relation to how fishing-induced mortality is affecting the global population (3).

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Glossary

Fish, an unpaired fin on the under surface of a fish, behind the anus.

Bycatch

In the fishing industry, the part of the catch made up of non-target species.

Caudal fin

The tail fin of a fish.

Crustaceans

A diverse group of animals with jointed limbs and a hard external skeleton, characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.

Dorsal fin

The unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.

Larvae

Stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.

Molluscs

A diverse group of invertebrates, mainly marine, that have one or all of the following; a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.

Parasite

An organism that derives its food from, and lives in or on, another living organism at the host’s expense.

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